Electrochemical machining using a chloride electrolyte including an alkali metal styrene sulfonate



United States Patent 3,404,077 ELECTROCHEMICAL MACHINING USING A CHLORIDE ELECTROLYTE INCLUDING AN ALKALI METAL STYRENE SULFONATE Warren R. Doty, Royal Oak, and Mitchell A. La Boda,

East Detroit, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Oct. 23, 1965, Ser. No. 504,110 19 Claims. (Cl. 204-143) ABSTRACT OF THE DISCLOSURE An electrochemical machining electrolyte consisting essentially of an aqueous solution of an electrolyte and an electrochemical erosion-inhibiting film-forming additive from the group consisting of alkali metal salts of a sulfonated styrene wherein the preferred concentration of the sulfonated styrene is from about 6.0 to about 14.3% by weight.

This invention relates to electrochemical machining processes and, more particularly, to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes, cavities and contoured surfaces have been developed and are more generally classified into one of two basic categories, the first being electrochemical machining and the second electrolytic grinding, a specialized application of the first. Electrolytic grinding is essentially an electrochemical deplating process which can be used on virtually any electrically conductive material. It is generally adapted to metal removal operations comparable to those performed by cutoff wheels, saws, and grinding or milling machines and the like, and uses equipment similar to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action.

A particular version of an electrolytic grinding process is characterized by a flow of electrolyte between the Workpiece and a rotating grinding cathode wheel. The rotating cathode wheel comprises a conductive metal matrix having a plurality of nonconducting abrasive particles imbedded therein to provide nonconductive spacing between the workpiece and the cathode matrix. Electric current is passed through the electrolyte to dissolve the anodic surfaces of the workpiece and the imbedded particles of the wheel abrade the surface to remove any irregularities resulting from nonuniform erosion or reaction product build up.

While aqueous solutions of individual inorganic salts, such as nitrates, cyanides, carbonates, hydroxides and nitriles have been used as electrolytes in electrochemical machining and grinding processes, none has oifered any significant advantage over the now well accepted aqueous sodium chloride solution most commonly used today. However, regardless of what salt is chosen, an apparent problem with electrochemical machining and grinding processes using that salt singularly or in combination is overcut, which is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. Such anodic dissolution can occur even in areas which are fairly well removed from the cathode. The overcut or cutting in low current density areas which are bathed in the electrolyte but substantially removed from the cathode, has been substantially reduced in the prior art by the use of costly and time-consuming masking operations which isolate the areas to be machined by protecting the surrounding areas from the erosive cifect of the electrolyte. These masking operations are frequently quite involved and require a high degree of skill to insure a satis- "ice factory product. Likewise, additional steps subsequent to the machining steps are required to strip the workpiece of the mask. Additionally, the prior art has attempted to reduce overcut by designing special purpose electrodes and machines to meet individual and specialized machining requirements.

By our invention we have at least reduced, and in most cases actually eliminated, the need for recourse to the prior arts attempted resolutions.

It is, therefore, an object of our invention to provide a self-masking electrolyte for ECM.

It is a further object of our invention to provide an additive for existing ECM electrolytes which selectively inhibits anodic dissolution in unwanted areas.

It is a further object of our invention to effect a sharply contoured machining by a process utilizing an aqueous electrolyte containing known salts and improving same by adding thereto acompound which upon reaction with the work-piece forms a film thcreover, which film inhibits or stops-off electrolytic action or anodic dissolution in unwanted areas.

It is a further object of our invention to effect a sharply contoured electrochemical machining by a process utilizing aqueous electrolytes containing known salts and an additive consisting of an alkali metal salt of a sulfonated styrene, and particularly sodium styrene sulfonate.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

The invention briefly stated involves adding an alkali metal salt of a sulfonated styrene to known ECM electrolytes. When added to these electrolytes the additives of this invention, and especially sodium styrene sulfonate create an inhibited solution which effectively forms a heavy adherent film over the surface of the workpiece. The film retards or substantially eliminates electrochemical erosion in those areas protected by the film. In a particular application of the invention, a chloride electrolytic grinding electrolyte is modified by adding thereto sodium styrene sulfonate. The film formed is subsequently abraded away in those areas where electrochemical machining is to continue, hence presenting a limited uninhibited surface to unrestricted electrochemical action.

Our experience has been that using the additives of our invention, we can successfully machine metal samples ranging from the softer low carbon steels (SAE 1008) to the harder low alloy steels (SAE 5160H). However, the additive of our invention will be likewise applicable and effective for ferrous alloys wherein the iron content is or more.

While the preferred electrolyte comprises a solution of 12.0%, by weight, of sodium styrene sulfonate, or about 10.7% by weight of the styrene sulfonate radical, 8.0%, by weight, of potassium chloride and the balance water, I have found that effective electrolytes can be compounded using from as low as 6.0%, by weight, sodium styrene sulfonate or about 5.33% by weight of the styrene sulfonate radical to as high as 14.3%, by weight, of sodium styrene sulfonate or about 12.7% by weight of the styrene sulfonate radical the balance being a selected concentration of an aqueous alkali metal chloride solution wherein the concentration of the chloride may vary from dilute to saturated. Generally speaking, as the concentration of the styrene sulfonate increases so should the CI in order to maintain satisfactory solution conductivity. In this connection the lighter alkali metal (lithium, sodium and potassium) chlorides are preferred because they produce relatively neutral pHs, do not plate out or have a deleterious elfect upon the cathode, and represent a source of inexpensive material. It is significant to note that at the higher concentrations, it is desirable to mix the respective cations in order to avoid the problems associated with the common ion effect.

These electrolytes appear to operate satisfactorily at voltages up to 40 volts and current densities between and 250 amperes per square inch. However, current density does not appear to be a limiting factor and higher current densities could therefore be used.

Generally speaking, tests were conducted utilizing a system wherein steel tube samples were brought up to a rotating sintered bronze diamond impregnated wheel. A gravity feed system kept the samples at the face of the Wheel at all times. The feed system was such that an adjustable weight provided the capability of varying the pressures at which the samples would engage the wheel. It was found that to properly evaluate the inhibitive effects of my additives a minimum workpiece-to-wheel pressure should be employed in order to reduce the mechanical cutting component of the abrasive wheel. A room temperature electrolyte was pumped at a pressure of 9 p.s.i. through a bore in the workpiece and into the gap between the cathode and the workpiece at a rate of 0.25 gallon per minute. This gap was held constant by the spacer effect of the nonconductive diamond chips. Tube length decrease per unit time was used to determine metal removal rates. The tube ends were compared with those produced by electrochemically grinding similar samples under the same conditions with additive-free electrolytes. Similar tests were conducted using non-tubular stock meterial.

The following are some specific examples encompassed within the scope of the invention:

EXAMPLE 1 An electrolyte comprising 12.0%, by weight, of sodium styrene sulfonate, 8.0%, by weight, of potassium chloride and the balance water was used to machine a sample of an SAE 516OH alloy. A potential of 4.5 volts was applied and a current density of 75 amperes per square inch was maintained, resulting in a metal removal rate of 0.007 inch per minute. The sample exhibited a well defined closely controlled machining with a fine surface finish resulting from the formation of an excellent inhibiting film.

EXAMPLE 2 An electrolyte comprising 6.0%, by weight, of sodium styrene sulfonate, 8.6%, by weight, of sodium chloride and the balance water was used to machine a sample of an SAE 5160H alloy. The temperature of the electrolyte was maintained at 72 F. and the potential at volts. A current density of 75 amperes per square inch was maintained, resulting in a metal removal rate of 0.009 inch per minute. The sample exhibited square well defined edges with an adherent film over the nonmachined surfaces.

EXAMPLE 3 An electrolyte comprising 14.3%, by Weight, of sodium styrene sulfonate, 14.3%, by Weight, sodium chloride and the balance water was used to machine a sample of an SAE 51601-1 alloy. The temperature of the electrolyte was maintained at 72 F. and the potential at 10 volts. A current density of 200 amperes per square inch was maintained, resulting in a metal removal rate of 0.021 inch per minute. A strong adherent inhibiting film was formed thereby contributing to the production of a machining exhibiting square well defined edges.

It is appreciated that other than abrasive means for the local removal of the inhibiting film or our invention may be employed. Among the possibilities might be 10- calized breakdown under high current densities, washing away with increased electrolyte flow, and/ or a variety of sophisticated variations of these and others. Therefore, though the invention has been described in terms of certain preferred embodiments, it is to be understood that others may be adapted and that the scope of the invention is not limited except by the appended claims.

We claim:

1. A process for electrochemically machining a metal comprising the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathode electrode adjacent to but closely spaced from said metal so as to form a gap therebetween, flowing through said gap an aqueous solution consisting essentially of an electrolyte and an electrochemical erosion inhibiting film forming additive from the group consisting of the alkali metal salts of a sulfonated styrene, passing electric current through said cell and removing from selected areas the electrochemical erosion inhibiting film formed whereby electrochemical machining can continue in said areas.

2. The process as claimed in claim 1 wherein said additive is sodium styrene sulfonate.

3. The process as claimed in claim 2 wherein said electrolyte consists essentially of at least one alkali metal chloride.

4. The process as claimed in claim 3 wherein the concentration of said sodium styrene sulfonate is about 6.0 to about 14.3% by weight.

5. The process as claimed in claim 3 wherein the concentration of said sodium styrene sulfonate is about 12% by weight.

6. The proces as claimed in claim 3 wherein said metal is an iron and its alloys.

7. The process as claimed in claim 6 wherein said alloy is steel.

8. The process as claimed in claim 6 wherein the concentration of said sodium styrene sulfonate is about 6.0 to about 14.3% by weight.

9. The process as claimed in claim 6 wherein the concentration of said sodium styrene sulfonate is about 12% by weight.

10. The process as claimed in claim 8 wherein said alloy is steel.

11. The process as claimed in claim 9 wherein said alloy is steel.

12. The process as claimed in claim 1 wherein the concentration of said additive is such as to produce a sulfonate styrene radical concentration of about 5.33 to about 12.7% by weight.

13. The process as claimed in claim 12 wherein the concentration of said additive is such as to produce a sulfonated styrene radical concentration of about 10.7% by weight.

14. The process as claimed in claim 12 wherein said electrolyte consists essentially of at least one alkali metal chloride.

15. The process as claimed in claim 13 wherein said electrolyte consists essentially of at least one alkali metal chloride.

16. An electrochemical machining electrolyte con'sisting essentially of an aqueous solution of at least one alkali metal chloride and an additive from the group consisting of alkali metal salts of a sulfonated styrene wherein the concentration of said additive is such as to produce a sulfonated styrene radical concentration of about 5.33 to about 12.7% by weight.

17. An electrolyte as defined in claim 16 wherein said sulfonated styrene salt is sodium styrene sulfonate.

18. An electrolyte a's defined in claim 17 wherein the concentration of said sulfonated styrene salt is about 6.0 to about 14.3% by weight.

19. An electrolyte as defined in claim 17 wherein the concentration of said sulfonated styrene salt is about 12% by weight.

References Cited UNITED STATES PATENTS 2,939,825 6/1960 Faust et al. 204-l42 3,058,895 10/1962 Williams 204-143 3,130,138 4/1964 Faust et al 204-143 3,284,327 11/1966 Maeda et al. 204143 ROBERT K. MIHALEK, Primary Emmiuer. 

